Carboxylic Acids for IGCSE WJEC Chemistry | IGCSE WJEC 化学：羧酸考点精讲

📚 Carboxylic Acids for IGCSE WJEC Chemistry | IGCSE WJEC 化学：羧酸考点精讲

Carboxylic acids are a homologous series of organic compounds containing the functional group –COOH. In the IGCSE WJEC Chemistry specification, understanding their structure, naming, physical properties, and characteristic reactions—including acidity, neutralisation, and esterification—is essential. This article distils the key points you need to master for the exam, with paired English–Chinese explanations to reinforce your learning.

羧酸是一类含有官能团 –COOH 的同系物。在 IGCSE WJEC 化学考试大纲中，掌握羧酸的结构、命名、物理性质及其特征反应——包括酸性、中和反应和酯化反应——至关重要。本文提炼了你必须掌握的考点，并采用中英对照讲解，帮助你巩固学习。

1. Functional Group and General Formula | 官能团与通式

The functional group of carboxylic acids is the carboxyl group, –COOH. This group consists of a carbonyl (C=O) and a hydroxyl (–OH) attached to the same carbon atom. The general formula for non-cyclic carboxylic acids with one carboxyl group is CₙH₂ₙ₊₁COOH, where n starts from 0 for methanoic acid.

羧酸的官能团是羧基(–COOH)，它由一个羰基(C=O)和一个羟基(–OH)连接在同一个碳原子上构成。含有一个羧基的非环羧酸通式为 CₙH₂ₙ₊₁COOH，其中 n 从 0 开始对应甲酸。

The simplest carboxylic acid is methanoic acid (HCOOH), where the R group is just a hydrogen atom. Ethanoic acid (CH₃COOH) is the next member and the most common example used in school laboratories.

最简单的羧酸是甲酸（HCOOH），此时 R 基团仅为氢原子。下一个成员是乙酸（CH₃COOH），也是学校实验室中最常用的例子。

2. Naming Carboxylic Acids | 羧酸的命名

Carboxylic acids are named by replacing the final ‘-e’ of the corresponding alkane with ‘-oic acid’. The carbon of the carboxyl group is always counted as carbon number 1. For example, methanoic acid (1 carbon), ethanoic acid (2 carbons), propanoic acid (3 carbons), and butanoic acid (4 carbons).

羧酸的命名是将对应的烷烃名称词尾的“-e”替换为“-oic acid”。羧基中的碳原子总是编号为 1。例如，甲酸（1 个碳）、乙酸（2 个碳）、丙酸（3 个碳）和丁酸（4 个碳）。

When substituent groups are present, the position is indicated by a number, with the carboxyl carbon always being position 1. For instance, 2-methylpropanoic acid has a methyl group on the second carbon of a three-carbon chain.

当含有取代基时，用数字标明位置，羧基碳始终为 1 号位。例如，2-甲基丙酸是在三碳链的第二个碳上带有一个甲基。

3. Physical Properties | 物理性质

Short-chain carboxylic acids (up to four carbons) are colourless liquids with sharp, pungent smells. Ethanoic acid is the main acidic component of vinegar. Carboxylic acids have higher boiling points than similar-sized alkanes, alcohols, and even aldehydes/ketones because they can form strong hydrogen bonds and exist as dimers through two hydrogen bonds between two carboxyl groups.

短链羧酸（最多四个碳）是有刺激性气味的无色液体。乙酸是醋的主要酸性成分。羧酸的沸点高于相似分子量的烷烃、醇甚至醛酮，因为它们能形成强氢键，并且可通过两个羧基间的两对氢键形成二聚体。

Solubility decreases as the carbon chain length increases. Methanoic, ethanoic, and propanoic acids are fully miscible with water due to hydrogen bonding with water molecules. Longer-chain carboxylic acids become more hydrophobic and are essentially insoluble.

随着碳链增长，羧酸的水溶性下降。甲酸、乙酸和丙酸因能与水分子形成氢键而完全溶于水。长链羧酸疏水性增强，几乎不溶于水。

4. Acidity and the Carboxyl Group | 酸性与羧基

Carboxylic acids are weak acids; they only partially ionise in aqueous solution. The acidic proton comes from the –OH part of the carboxyl group. The negative charge on the carboxylate ion (RCOO⁻) is delocalised over the two oxygen atoms, stabilising the ion and making the acid more willing to release the proton.

羧酸是弱酸，在水溶液中只能部分电离。酸性质子来自羧基中 –OH 的部分。羧酸根离子(RCOO⁻)的负电荷离域在两个氧原子上，使离子稳定，从而使羧酸更容易释放质子。

In water, ethanoic acid establishes an equilibrium: CH₃COOH ⇌ CH₃COO⁻ + H⁺. The equilibrium lies well to the left. The acid dissociation constant Kₐ is small, typically of the order of 10⁻⁵ mol dm⁻³ for ethanoic acid.

在水中，乙酸建立如下平衡：CH₃COOH ⇌ CH₃COO⁻ + H⁺，平衡强烈偏左。酸解离常数 Kₐ 很小，乙酸的数量级约为 10⁻⁵ mol dm⁻³。

5. Reaction with Metals | 与金属的反应

Carboxylic acids react with reactive metals such as magnesium, zinc, and iron to produce a salt (the metal carboxylate) and hydrogen gas. This is a typical metal–acid reaction, albeit slower than with strong mineral acids because the concentration of H⁺ is lower.

羧酸可与活泼金属（如镁、锌和铁）反应，生成对应的羧酸盐和氢气。这是典型的金属与酸的反应，只是因为 H⁺ 浓度较低，反应速率比强无机酸慢。

Example: 2CH₃COOH(aq) + Mg(s) → (CH₃COO)₂Mg(aq) + H₂(g). Bubbles of hydrogen gas are observed and the metal dissolves.

例如：2CH₃COOH(aq) + Mg(s) → (CH₃COO)₂Mg(aq) + H₂(g)。可观察到氢气泡产生且金属溶解。

6. Reaction with Bases and Alkalis | 与碱和强碱的反应

Carboxylic acids undergo neutralisation with metal oxides, hydroxides, and carbonates. With sodium hydroxide, a soluble carboxylate salt and water are formed: CH₃COOH + NaOH → CH₃COONa + H₂O. This reaction is essentially a proton transfer from the acid to the hydroxide ion.

羧酸可与金属氧化物、氢氧化物以及碳酸盐发生中和反应。与氢氧化钠反应生成可溶性羧酸盐和水：CH₃COOH + NaOH → CH₃COONa + H₂O。该反应本质上是质子从酸转移到氢氧根离子。

With copper(II) oxide, a typical base, ethanoic acid produces copper(II) ethanoate and water: 2CH₃COOH + CuO → (CH₃COO)₂Cu + H₂O. The black solid dissolves to give a blue-green solution.

乙酸与典型碱氧化铜反应，生成乙酸铜和水：2CH₃COOH + CuO → (CH₃COO)₂Cu + H₂O。黑色固体溶解，形成蓝绿色溶液。

7. Reaction with Carbonates and Hydrogencarbonates | 与碳酸盐和碳酸氢盐的反应

A key test for carboxylic acids (and acids in general) is their reaction with carbonates or hydrogencarbonates to produce carbon dioxide gas. This can be identified by bubbling the gas through limewater, which turns milky.

检验羧酸（以及一般酸）的一个关键反应是它们与碳酸盐或碳酸氢盐反应产生二氧化碳气体。将气体通入石灰水，石灰水变浑浊即可确认。

Equation: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂. With hydrogencarbonate: CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂. The fizzing is vigorous and immediate.

反应方程式：2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂。与碳酸氢钠反应：CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂。可观察到剧烈的冒泡现象。

8. Esterification | 酯化反应

Carboxylic acids react with alcohols in the presence of a strong acid catalyst (commonly concentrated sulfuric acid) to form esters and water. This is a reversible condensation reaction, known as esterification. The ester functional group is –COO–.

羧酸与醇在强酸催化剂（常用浓硫酸）作用下反应，生成酯和水。这是一个可逆的缩合反应，称为酯化反应。酯的官能团是 –COO–。

Example: ethanoic acid + ethanol ⇌ ethyl ethanoate + water. The reaction mixture is usually heated under reflux, and the ester is a volatile liquid with a characteristic sweet, fruity smell.

例如：乙酸 + 乙醇 ⇌ 乙酸乙酯 + 水。反应混合物通常需加热回流，酯是一种具有特征果香味的易挥发液体。

In the IGCSE WJEC exam, you may be asked to identify the products, write balanced equations, and describe laboratory preparation. You should know that the acid catalyst speeds up both the forward and reverse reactions equally so it does not affect the position of equilibrium.

在 IGCSE WJEC 考试中，你可能需要鉴定产物、书写配平方程式并描述实验室制备方法。你还要知道，酸催化剂同等程度地加快正逆反应速率，因此不影响平衡位置。

9. How to Identify a Carboxylic Acid | 如何鉴别羧酸

The simplest laboratory test for a carboxylic acid is to add a small spatula of solid sodium carbonate (or sodium hydrogencarbonate) to an aqueous solution of the unknown. Immediate effervescence indicates the presence of a strong enough acid to react with the carbonate. The gas produced turns limewater milky, confirming it is carbon dioxide.

实验室中鉴别羧酸最简单的方法是，向未知物的水溶液中加入一小勺固体碳酸钠（或碳酸氢钠）。立即产生大量气泡表明存在足够强的酸性物质能与碳酸盐反应。生成的气体使石灰水变浑浊，证明是二氧化碳。

Because phenols are much weaker acids, they will not effervesce with carbonates. This differentiates carboxylic acids from phenols. However, you can also smell the characteristic vinegary odour for lower carboxylic acids, though safety precautions must be taken.

因为酚的酸性弱得多，它们与碳酸盐不产生气泡，这可以区分羧酸与酚。此外，低级羧酸具有特征的醋酸气味，但在进行气味鉴别时须注意安全。

10. Carboxylate Salts and Soaps | 羧酸盐与肥皂

Long-chain carboxylic acids (fatty acids) can be neutralised by sodium hydroxide to form sodium salts, which are soaps. The general equation is: RCOOH + NaOH → RCOONa + H₂O, where R is a long hydrocarbon chain, typically 12–18 carbons.

长链羧酸（脂肪酸）可被氢氧化钠中和生成钠盐，这就是肥皂。通式为：RCOOH + NaOH → RCOONa + H₂O，其中 R 是长碳氢链，通常含有 12–18 个碳。

These soaps possess a hydrophilic (–COO⁻) head and a hydrophobic hydrocarbon tail. This dual nature allows soaps to emulsify oils and grease in water, making them useful cleaning agents. This topic links to broader discussions of surfactants and detergents.

这类肥皂具有亲水的(–COO⁻)头部和疏水的碳氢尾部。这种双重性质使肥皂能将油脂在水中乳化，成为有用的清洁剂。该主题还可与表面活性剂和洗涤剂的讨论联系起来。

11. Summary of Key Reactions | 关键反应总结

For exam success, memorise the following reaction patterns (using ethanoic acid as a model):

为在考试中取得好成绩，请记住以下反应模式（以乙酸为准）：

With metal: 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂
With alkali: CH₃COOH + NaOH → CH₃COONa + H₂O
With carbonate: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂
Esterification: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O (acid catalyst, heat)

You should be able to write these equations using displayed formulae, showing all bonds. Practice writing equations for other carboxylic acids like propanoic acid to build confidence.

你应该能用全显示式写出这些方程式，展示所有化学键。多练习书写丙酸等其他羧酸的反应方程式，以巩固信心。

12. Common Exam Pitfalls | 常见考试易错点

One common mistake is confusing the carboxyl group with the carbonyl or hydroxyl groups individually. The –COOH functional group must be drawn and written correctly; the carbon is double-bonded to one oxygen and single-bonded to an –OH group. Many students mistakenly write COOH as CO₂H or miss the double bond.

常见的错误是混淆羧基与单独的羰基或羟基。–COOH 官能团的书写和绘制必须准确：碳原子与一个氧原子以双键相连，并与一个 –OH 基团以单键相连。很多学生错误地把 COOH 写成 CO₂H 或遗漏双键。

Another pitfall is forgetting that carboxylic acids are weak acids. In equilibrium equations, the ⇌ symbol must be used, not a single arrow. Also, when writing equations for esterification, always remember to include the water molecule as a product and to write the ester functional group as –COO–, not –C–O–O–.

另一个易错点是忘记羧酸是弱酸。在平衡方程式中必须使用 ⇌ 符号，而非单箭头。此外，书写酯化方程式时，务必记得产物中有水分子，酯的官能团应写作 –COO–，而不是 –C–O–O–。

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Carboxylic Acids for IGCSE WJEC Chemistry | IGCSE WJEC 化学：羧酸 考点精讲